Some validation error messages are not propagated

[nicoweidner]

One example: Annotation validation does not return anything, but the place that calls it expects a return value. There may be other examples as well.

In general, I would suggest to

• either pass a mutable object, mutate it in place and not return anything
• or just return a new object that can then be used (I would prefer the second option)

Shameless plug: Python type hints could have helped catch this.